The process of assembling printed circuit boards (PCBs) generally requires solder paste to be applied to the printed circuit board before components, such as integrated circuits and discrete devices, are placed and ultimately secured onto the board. Initially, the sticky solder paste temporarily holds the components in place on the PCB until the solder paste is melted or reflowed. When the solder paste is reflowed it forms both mechanical and electrical connections between the components and the board. If there is not enough solder paste, components may be inadequately mechanically and/or electrically connected to the PCB. If there is too much solder paste or if solder paste is deposited in the wrong place, an extra or incorrect electrical connection such as a short circuit may result, generally referred to as a "solder bridge".
Automated processes for fabricating PCBs are generally known, including process steps for applying solder paste to printed circuit boards using screen printing techniques. Prior to printing, a thin metal stencil or template is constructed with openings or apertures matching appropriate places on a particular PCB where paste should be printed. During printing the stencil is aligned to the PCB. The solder paste is then applied to the PCB through the stencil openings, typically using a squeegee to spread the paste. Then the stencil is lifted off of the PCB which is transported to its next manufacturing process step.
Automatic inspection or vision systems may be used in known processes to detect problems in the printing process, including problems associated with the stencil and problems associated with the application of paste to the PCBs. Stencil blockage may be detectable wherein dried solder paste or debris causes stencil openings to clog. It is desirable to detect stencil blockage by inspecting the stencil before any printed circuit boards are actually misprinted with solder paste. Stencil smearing may be detectable wherein solder paste smears around the stencil openings on a side of the stencil adjacent to the PCB. It is also desirable to detect stencil smearing by inspecting the stencil before any boards are actually misprinted with solder paste. Misalignment of the stencil and PCB may be detectable prior to distribution of solder paste by inspection of the stencil position relative to the PCB before printing thereon. The PCB can be inspected to detect incorrect solder paste distribution after printing. Automatic inspection may be used to determine if there is too little solder paste, too much solder paste, or solder paste in the wrong place on the printed circuit board.
The automated inspection of solder paste on PCBs is inherently difficult to accomplish. The solder paste is difficult to identify on the PCB because it is variable in terms of its appearance. The paste appearance varies over time, e.g. wet paste has a different appearance than dry paste. The paste has texture and the gray level reflectance of the paste is similar to solder pads with tinning and other circuitry on the PCB. Also, the three dimensional shape of the paste causes shadowing, making automated inspection difficult. The inherent difficulty is exacerbated by variability from one PCB to the next. For example, the color of boards of a single board type in one printing run can vary from light green to dark green or blue. In one printing run, the relative positioning of portions or all of a PCB of a single board type may vary with respect to the stencil or solder mask on the board. Additionally, all of the circuitry on a board is subject to dimensional variability. Perhaps most significantly, the solder paste deposited on a board can obscure solder pads beneath it making it impossible to tell either what is beneath the solder paste or what is the relative alignment of the solder paste to what is beneath it. For instance, if the solder paste is exactly the same size and shape as the solder pad beneath it, it may be impossible to tell whether there is perfect coverage or no coverage at all.
Known in-line or image automated inspection systems for solder paste inspection do not adequately overcome such difficulties. Some known systems typically only perform inspection based on an image of the PCB captured after the solder paste has been deposited thereon (referred to hereinafter as "post-only" systems). Known post-only systems typically use a vision/inspection tool that assigns grey scale values to pixels of a captured image of a PCB under inspection after printing. Pixels are classified as being not-paste or paste based on their grey scale value. Pixels are aggregated for particular areas of interest on the PCB, in a connectivity analysis in which pixels connected to one another and identified as being paste are labelled as belonging to the same lump of paste. Characteristics of interest related to the aggregation of pixels, such as area, center of mass, and size of enclosing rectangle, are determined and ultimately used by an operator to adjust parameters of the printing process to maintain the post-printing product within acceptable tolerances.
Unfortunately, color variation in the underlying areas of the PCB negatively affects the inspection process because grey scale values will vary according to the color variation of the underlying portions of the PCB, especially near the paste edge where the solder paste is thin. Variability in color/composition of the solder paste may also cause grey scale variations that negatively affect the inspection process. Similarly, known post-only systems may be negatively affected by lighting, which must be rigorously controlled in order to avoid negative affects on the inspection process due to shading or reflection of light from components of the system. In general, the yield and accuracy of known post-only solder inspection systems are not sufficient to make such systems useful or practicable.
Many known solder paste inspection systems are stand-alone (i.e. not "in-line") stations dedicated to performing solder paste inspection apart from and subsequent to the screen printing process step(s). Such stand-alone systems include optics and lighting that are optimally configured for the inspection of solder paste. The optics and lighting dedicated to solder paste inspection represent costly, redundant hardware in the PCB fabrication process flow in that similar optics and lighting are necessary upstream in the process flow to effect registration or alignment in the screen printing stage. Furthermore, stand-alone solder paste inspection systems typically incorporate laser stripers and other very expensive and sophisticated hardware that is often difficult to justify in a cost/benefit analysis. Stand-alone systems disadvantageously introduce additional machine set-up time and maintenance, require additional factory floor space to implement, and introduce additional undesirable handling of PCBs to convey them through the stand-alone inspection stage. Generally, a stand-alone solder inspection system will represent a significant overall reduction in throughput in a PCB fabrication line.